Ultrasonic inspection apparatus using certain polymeric materials as couplants

ABSTRACT

An apparatus is disclosed for making ultrasonic inspections of high surface temperature objects by using as a couplant a polymeric material of polysulfone or polyester resin having specifically defined thermal characteristics. Surface temperatures of the object at the point of inspection may be 300* - 1,100* F. or higher.

United States Patent 1 Rathburn et al.

[451 July 24,1973

[ ULTRASONIC INSPECTION APPARATUS USING CERTAIN POLYMERIC MATERIALS AS COUPLANTS [75] Inventors: Richard P. Rathburn, Richmond;

Garth M. Stanton, San Anselmo, both of Calif.

[73] Assignee: Chevron Research Company, San

' Francisco, Calif.

[22] Filed: Sept. 20, 1971 [21] Appl. No.: 181,789

Related US. Application Data [63] Continuation-impart of Ser. No. 809,029, March 20, 1969, abandoned, and a continuation-in-part of Ser. No. 20,069, March 16, 1970, Pat. No. 3,606,791.

[56] References Cited UNITED STATES PATENTS 3,606,791 9/1971 Rathburn et al 73/71.5 3,393,331 7/1968 Puckett 3,427,481 2/1969 Lenahan et al. 73/71.5 UX

Primary Examiner- Richard C. Queisser Assistant Examiner-John P. Beauchamp Attorney-A. LfShdWQC. J. To nkin et al.

[57] ABSTRACT An apparatus is disclosed for making ultrasonic inspections of high surface temperature objects by using as a couplant a polymeric material of polysulfone or polyester resin having specifically defined thermal characteristics. Surface temperatures of the object at the point of inspection may be 300 l,100 F. or higher.

4 Claims, 1 Drawing Figure MEASUREMENT INDICATOR SYSTEM PAIENInJuL24|91s ULTRASONIC INSPECTION APPARATUS USING CERTAIN POLYMERIC MATERIALS AS COUPLANTS CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of application Ser. No. 809,029, filed Mar. 20, 1969, now abandoned and application Ser. No. 20,069, filed March 16, 1970, now US. Pat. No. 3,606,791 issued Sept. 21, 1971.

BACKGROUND OF THE INVENTION This invention relates to an apparatus for transmitting ultrasonic waves into solid materials. More particularly, it relates to such apparatus for transmitting waves into solid materials having high surface temperatures.

In recent years, ultrasonic apparatus for nondestructively examining and measuring various properties of solid materials have become extensively used. Ultrasonic apparatus may be used in measuring thicknesses of solid materials such as steel ship hulls, oil refinery reaction vessels, piping or columns, or plates or beams of materials such as those used in aircraft. Ultrasonic examination is also used to detect flaws in the interior of solid materials such as thick steel plates or welds.

In a typical ultrasonic examining apparatus, one employs a transducer containing an ultrasonic wave generating source, a receiving device to receive the ultrasonic wave reflected from walls or other surfaces of the solid body being examined, and some sort of measurement-indicating device to indicate to the operator the particular quantity being measured, which may be, for instance, the depth or width of a flaw within the solid object being examined. The receiver may be part of the transducer assembly or it may be a separate device. In practice, the ultrasonic waves generated by the transducer pass through the transducer face into the solid material and are reflected from various surfaces within the material. These surfaces may be flaws within the material, welded joints abutting the material, or simply outer surfaces of the material. The reflected wave is detected by the receiver and the signal generated in the receiver upo'n detection of the wave is passed to the indicating device where it is converted into a signal from which the operator can determine the particular quantity or measurement he seeks. Most often the indicating device is an oscilloscope and the reflected wave signal appears as a peak on the oscilloscope screen. Alternatively, the indicating device signal may appear as the deflection of a needle on a meter, a line on a graph, or even a series of numbers on a printed sheet where the receiving device is connected to a digital output device.

It is found in practice that because of the normal irregularities of the surfaces of the object to be examined and the transducer face, good contact between the transducer face and the object usually cannot be obtained simply by pressing the transducer face against the object. While there will be a number of points of contact between their two surfaces, there will also be numerous points where the surfaces do not touch, thus leaving air gaps between the two surfaces. These air gaps prevent smooth transmission of the ultrasonic waves from the transducer face into the solid body and cause the reflected wave pattern detected by the receiver to be extremely distorted, thus preventing the operator from obtaining satisfactory data from his measurements.

To overcome this problem, couplants are used in ultrasonic examination. A couplant is a relatively deformable material which is placed between the surfaces of the transducer and the solid material. The couplant fills substantially all the spaces between the two surfaces and essentially eliminates the troublesome air gaps. The ultrasonic waves pass smoothly from the transducer face, through the couplant, and into the solid body; and where the receiver is an integral part of the transducer, the reflected waves pass smoothly out of the solid material, back through the couplant, and into the transducer.

Most ultrasonic measurements are made on materials which are at ambient or slightly elevated temperatures. These slightly elevated temperatures may be as high as about 300 F. At ambient temperatures and temperatures up to about 200 F., a number of common materials such as water and light lubricating oil make satisfactory couplants. At higher temperatures, up to about 300 F., more viscous materials such as petroleum and silicone greases have been used.

Above 300 F., and especially above 400 F., however, these materials are unsatisfactory for a number of reasons. Some, such as water, boil off or evaporate rapidly, while others such as the greases become extremely fluid and run. These obviously cannot be used on vertical or overhead surfaces. Further, many develop gas bubbles within the couplant and these gas bubbles distort the ultrasonic transmission in much the same way as do air gaps. Consequently, in most cases, ultrasonic inspection processes hav been limited to applications at temperatures below about 300 F.

This low temperature limitation has not particularly inhibited use of ultrasonic inspection apparatuses in a number of fields, such as aircraft construction and some phasesof ship construction. It has, however, severly limited their usefulness in such fields as oil refin-' ing, chemical production, weld inspection, and fabrication of heavy metal plates and beams. This is because many operations in these fields occur at high temperatures, i.e., temperatures above about 300 F. and especially above about 400 F. to about ll00 or l200 F. Typical are oil refinery processes in which reaction vessels, transfer pipes, furnaces, and much other metal apparatus operate at these higher temperatures.

Operators of such high-temperature processes have, for many years, sought a method whereby the hightemperature apparatus could be inspected for flaws, metal lost by corrosion, and other defects, while remaining in service and being operated at the high temperatures. In most cases, such inspection has not been possible and the operator has been forced to remove the apparatus from service and cool it down to a temperature of 300 F. or lower to permit the ultrasonic inspection with conventional couplants. Obviously, this has been an unsatisfactory procedure because of the loss of operating time.

Three systems have been suggested of inspecting high-temperature apparatus while the apparatus is operating. In the first of these, water is continuously pumped through the transducer head into the gap between the transducer face and the high-temperature surface, serving both as a couplant and a coolant. The

' water turns to steam almost immediately upon contacting the hot metal surface, but if the water volume is great enough, the conversion to steam is delayed just long enough for relatively rough measurements to be obtained by the ultrasonic device. This method, however, is severely deficient in that it requires considerable extra apparatus to provide the water flow and a highly complex transducer head, incorporating means for the water to pass through. It also subjects the operator to the hazards of working in an atmosphere of hightemperature steam. Finally, on some materials the quenching effect of the water is undesirable.

In another system, it has been proposed to use molten metal or fused salt as the couplant at high temperatures. This has the obvious disadvantage of requiring special handling means to contain the molten metal or fused salt and to protect the operator from the serious hazards of working with these materials.

The third system proposed employs, as a couplant, an organic material. This is placed against the hot surface where it rapidly is burned to a substantially carbonaceous layer. The carbonaceous layer then serves as the couplant. This has been found to be relatively unsatisfactory, however, for it permits only one attempt by the operator to' obtain good transducer-to-metal surface contact. If the transducer face is not precisely placed against the carbonaceous couplant on the first attempt, the carbonaceous layer is disturbed to such an extent that it cannot further serve as a couplant. Consequently, if the operator does not place the transducer face against the couplant at precisely the correct angle on the first try, he cannot remove the transducer and make a second attempt. This is obviously a disadvantage, since in many cases ultrasonic measurements are made in surroundings where the operator must work in a cramped or awkward position, and so cannot be expected to place the transducer in position accurately on the first attempt.

DESCRIPTION OF THE PRIOR ART An article by Ostrofsky in Chemical Engineering, 75, l l, 174 (May 20, 1968) describes low-temperature ultrasonic measuring and briefly discusses .suggested means of making measurements at higher temperatures. U.S. Pat. No. 3,393,331 suggests that heat softening plastics," which are exemplified by polyethylene and nylon, may be used as high-temperature couplants. U.S. Pat. No. 3,394,586 discloses that a polysiloxane elastomer may be used as a delay line transmission element, and suggests that this material may also combine the functions of delay line element and couplant at temperatures uptol000 F. Other patents relating to ultrasonictesting include U.S. Pat. Nos. 2,830,201; 2,697,936; 2,846,875; 3,002,375; 3,320,797; 3,074,267; 3,242,723; 3,423,991; and 3,497,728.

Polysulfone resins are described in articles in Chemistry and Industry, p. 461 (Apr. 13, 1968) and The Modern Plastics Encyclopedia, I968 edition, p. 259, as well as in U.S. Pat. Nos. 3,225,104; 3,359,898; 3,380,878; and 3,408,437.

SUMMARY The present apparatus allows the making of hightemperature ultrasonic measurements rapidly and with considerable ease, safety to the operator, and reproducibility. Further, measurements may be made on nonhorizontal surfaces, such as vertical or overhead pipes or vertical reaction vessels without encountering the problems of couplants which run or drip from the spot onto which they are placed.

In summary, this invention is an apparatus for making an ultrasonic inspection of an essentially solid body from a surface point on that body where the surface point has a temperature designated T,; which apparatus comprises, in combination, a transducer operatively connected to an ultrasonic wave generating source, a receiving device to receive the reflections of the ultrasonic waves transmitted into the body at the surface point, a measurement-indicating device operatively connected to said receiving device, a delay line or signal conveying bar for spacing each transducer per se from said high temperature body being inspected, and disposed at the end of said delay line between said line and said body, a polymeric couplant material 1) having a glass transition temperature less than T, and a distortion temperature greater than both 300 F and T,,; and 2) which is deformable at T, to substantially fit the contours of both the transducer face and the surface point; and 3) T is greater than 300 F which polymeric couplant material is selected from the group consisting of polysulfone and polyester resins.

DRAWING The drawing is a block diagram schematically illustrating the invention.

DETAILED DESCRIPTION OF THE INVENTION In its broadest form, this invention is an apparatus for making an ultrasonic inspection of an essentially solid body from a surface point on that body where the surface point has a temperature designated T, whereby ultrasonic waves are transmitted from the face of a transducer through a couplant into the body at the surface point and undistorted reflections of the waves are detected after emerging from the body. In this combination, the couplant is a polymeric material 1) having a glass transition temperature less than T, and a distortion temperature greater than both 300 F. and T,; and 2) which is deformable at T, to substantially fit the contours of both the transducer face and the surface point; and the couplant material is a polysulfone or polyester resin of the foregoing characteristics. Preferred embodiments of this invention will be described below.

Referring to the drawing, the apparatus of the present invention for making an ultrasonic inspection of a relatively high surface temperature body 1 shown in section for which the thickness is to be determined, comprises a transducer 2 operatively connected to an ultrasonic wave generating source and suitable input controls 3, a receiver which is normallythe transducer operating as a receiver for the reflections of the waves, a measurement indicating device 4 operating with the receiver to display or otherwise indicate the reflection signals and a delay line or signal conveying bar 5 for As indicated, the principal improvement of this invention is the feature of using a specific type of polymeric material as a couplant. This type of polymeric material is defined by characteristics described in detail below. Its usefulness in the process of this invention is based on its ability to transmit ultrasonic waves without distortion at temperatures above 300 F. In a preferred embodiment, these materials transmit ultrasonic waves without distortion at temperatures above 400 F., and, in a more preferred embodiment, at temperatures above 600 F.

In the apparatus combination of this invention, the temperature of interest is the surface temperature of the solid body at the point at'which the body is to be examined ultrasonically. For convenience, this temperature will hereinafter be referred to as T,. In the broadest form of this invention, T is greater than 300 F. In a preferred embodiment, T, is greater than 400 F., and in a more preferred embodiment, greater than 600 F.

An important characteristic of the polymeric material is its deformability. In order to be useful, the polymeric material must be sufficiently deformable at T, that it substantially fits all the contours of both the transducer face and the surface of the solid body at the surface point of examination. It must fill the space between the body surface and the transducer face without creating any bubbles or gaps in the couplant itself. It must also adhere to the surface of the object being examined if that surface is rounded or nonhorizontal.

Directly related to the deformability of the polymeric material is its glass transition temperature. The glass transition temperature of a polymer is defined on page 26 of the 1968 edition of the Modem Plastics Encyclopedia (McGraw-Hill Publishing Company), as the temperature below which molecular chain motion in the polymer is frozen in. Above this temperature there is sufficient energy to permit motion and undulations in the polymer chain. Below the glass transition tempera ture the polymeric materials are stiff, hard and often brittle. i

In order to guarantee the deformability of the polymeric material it is required that the glass transition temperature of the polymeric material be less than T,. If the glass transition temperature of the polymeric material is higher than T,, the polymeric material will remain hard and cannot be deformed sufficiently to act as a satisfactory couplant.

Another principal characteristic required of the polymeric material is what will herein be termed the distortion temperature." This is defined as the temperature above which the polymeric material develops physical characteristics which distort the ultrasonic waves passing through the couplant and thus prevent the collection of reliable ultrasonic examination data. This occurrence of distortion-causing characteristics may take any one of several forms: most commonly the polymeric material melts, chars, or flames. The polymeric material is deemed to have melted when it becomes fluid enough to run relatively freely or when gaseous bubbles begin to form in the polymeric material. It is deemed to have charred when a layer of carbon has been formed of sufficient thickness such that the surface region of the polymeric material no longer has a tacky consistency. The flaming point is, of course, apparent. The occurrence of the distortion-producing characteristic, however, is not limited to the melting, charring, or flaming of the polymeric material. All

other forms of change of the physical characteristics of the polymeric material which cause distortion of the ultrasonic wave are intended to be included within this definition. This would include such occurrences as oxidation of the surface layer of the polymer where such oxidation causes distortion of the ultrasonic waves.

Operation of the process of this invention, as practiced at our direction by the Equipment Inspection Section of the Standard Oil Company of California Refinery at Richmond, Calif. is basically similar to the operation of processes for making ultrasonic measurements at temperatures below 300 F. Because the transducer itself must be protected from heat, however, a heat sink or delay line" is usually placed between the hot surface and the actual face of the transducer. The presence of this delay line does not affect the operation of the apparatus of this invention. Consequently, throughout this specification and in the claims, the term face of the transducer" will be understood to signify the face of the delay line where appropriate operational requirements call for use of a delay line.

In practice, the operator of the ultrasonic apparatus locates the exact surface point on the vessel, pipe, plate, or other solid object where he wishes to make his inspection. He places the polymeric couplant on the objects surface at that point. He then waits a short time, generally no more than a few seconds, for the heat from the surface to soften the couplant. (Because of the adhesive properties of the polymeric couplant, the operator may, if desired, wait for several minutes after the couplant softens before continuing with the examination). He then pushes the face of the transducer against the softened couplant. This forces the deformable couplant into the contours of both the transducer face and the objects surface, and completely fills the gap between the two. Ultrasonic waves generated by the transducer are then transmitted through the transducer face and couplant into the solid body.

The reflected waves are detected as they emerge from the body, either by the transducer itself (now acting as a receiver) or by a separate receiver in contact with the object through a second couplant. This second couplant may be a polymeric material if the surface temperature at the reception point is greater than 300 F. If it is a polymeric material, it may be the same or a different polymeric material as that which is in contact with the transducer. If the surfacetemperature at the reception point is less than 300 F., a conventional couplant may be used. The reflected wave is monitored by a visual display means which has usually been precalibrated to display the desired measurement directly. For instance, if an oscilloscope is used as the display means, the separation of reflected wave peak heights can be calibrated to indicate thickness of a pipe wall directly in inches or centimeters.

We have found it most convenient to use two men to operate this invention, although in many cases one man can easily perform the operations. When two men are used, the power source for the transducer and receiver and the display means are grouped in a central location (we have collected these in a mobile van) and the transducer and receiver are located at the inspection site remote from that central location but connected thereto by electrical cables. One man monitors the equipmnt at the central location while the other, encumbered only by the couplant, transducer, and receiver, and their trailing cables, makes the measurement at the desired inspection site. Since transducer/- receiver combinations housed in containers having external length, width, and depth each less than 1 inch are commercially available, and, as will be discussed below, the couplant is easily carried, the man making the actual inspection can easily work in confined spaces. Further, not being burdened with heavy or bulky equipment, he can often reach points (by climbing, crawling, etc.) that would be inaccessible to a man who was so burdened.

The polymeric materials may be used in many convenient forms. At ambient temperatures they are solid, and those commercially available are often in the form of large thin sheets. We have found it convenient to use the material in small strips. The actual size will depend on the largest dimension of the transducer face. We have used a transducer with a one-half inch diameter face; the couplant strips used in conjunction with this instrument were approximately 4-6 inches long and one-half inch wide.

Thickness of the polymeric material should be minimal, providing only enough thickness to fit the con- .tours of the transducer face and object surface and maintain a small, couplant-filled separation between them. Thicknesses should be in the range of about 0.00l-0.2 inch, with a 0.0040.l inch preferred.

Optimum overall size of the couplant for ease of use is based largely on the poor heat transfer characteristics of the polymeric material. Since heat from the object's surface flows only slowly through the material, the couplant should be thin to permit rapid softening of entire thickness of the couplant adjacent the surface inspection point. By having a piece of couplant several times longer than its width, however, the operator can comfortably hold one end of the couplant strip (which is cool) while the other end is in contact with the heated object surface.

Where the couplant is used in the form of a small rectangular thin strip, the operator can easily carry several strips ina pocket or pouch and can make numerous inspections before exhausting his supply of couplant. A single strip of couplant may often be reused several times if it does not adhere so strongly to an object surface'that it is torn apart'on attempted removal. As an alternative, the couplant, if it is sufficiently flexible at ambient temperatures, may be in the form of a long narrow thin strip, coiled such that the operator may pull off whatever length of material he needs for a particular inspection.

The polysulfone and polyester resin couplant materials as defined above have been tested and have been found satisfactory for use In this process. Each has atemperature range in the region above 300 F. at which 'it is most satisfactory for use. These polymers generally have number average molecular weights in the range of WOO-100,000. Suitable polyester resins for use as couplants in this .invention are the poly(ethylene terephthalate) polymer commercially available under the trademark Mylar. The polysulfone resins, which are preferred couplants for use in making ultrasonic measurements at above about 400 F., preferably 650 to ll F. or greater, are poly(arylene sulfones) having the monomer unit structure:

wherein R is a divalent hydrocarbon, halohydrocarbon, or oxahydrocarbon radical. (The oxahydrocarbon radical, as defined herein, is a radical containing only oxygen, hydrogen, and carbon atoms. The oxygen atom or atoms will be present only in ether, hydroxy, or carbonyl groups.) In a preferred form, R is a radical comprising two phenylene groups, linked by an alkylene, haloalkylene, or oxalkylene group. (The oxalkylene radical is a form of the oxahydrocarbon radical, and is an oxygen derivative of an alkylene radical). in a more preferred form, R is a hydrocarbon radical comprising two phenylene groups linked by an alkylene group having one to six carbon atoms.

As noted, R-represents a wide variety of hydrocarbon and substituted hydrocarbon radicals. A number of these are described in the literature. The more preferred structures are shown below in Table I. Also included in Table l is the glass transition temperature of the polymer containing each of the listed structures. The significance of the glass transition temperature, designated T,, was mentioned hereinabove.

Other representative R radicals less preferred, but suitable for use in this invention, are listed in Table 11 below. These are meant to be illustrative only, and not exclusive; any polysulfone resin which has the characteristics described in this specification will be suitable for use as a couplant.

(Ili

In order to guarantee the deformability of the polysulfone resin, it is required that its glass transition temperature be less than T,. If the glass transition temperature is higher than T,, the polysulfone resin will remain hard and cannot be deformed sufficiently to act as a satisfactory couplant. A number of the polysulfone resins such as that in which R is the radical will have glass transition temperatures higher than the 400 F. minimum T, noted above. These are included within the scope of this invention, but are limited to use in the preferred embodiments wherein T, is 600 or 700 F., depending on the particular polysulfone resin used.

Typical applications of the use of the apparatus of this invention are illustrated in Table III below as Examples 1 through 21. Each of these examples describes an ultrasonic measurement made on a particular piece of oil refinery process equipment. In most cases the purpose of the ultrasonic inspection was to detect flaws or cracks in refinery piping or to measure the wall thickness of piping to determine corrosion damage. Measurements were made using the inspection techniques described above. The polysulfone resin used as a couplant had as its R group the radical designated C in Table l above and the polysulfone resin was obtained commercially from the Union Carbide Company. Strips of polymeric material about 12 inches long by one-half inch wide by 0.05 inch thick were used.

In Table lll below, the particular type of refinery apparatus examined is described. The surface temperature at the point of inspection is given and the polymeric couplant used is also listed. Nominal diameters of the various pipes inspected ranged from 3 inches to 16 inches. The term FCC is an abbreviation for fluid catalytic cracker. All pieces of equipment listed were in full service while being examined.

TABLE lll Surface Polymeric Example Apparatus Examined Temp. 'Couplant "F. l Hydrocraeker furnace l 100 Polysulfone crossover lines 2-6 962 7 Coker outlet lines 950 Thermal cracker evaporator 780 column bottoms line Thermal cracker fractionator column side cut line 750 l FCC fractionator 650 Mylar bottoms line l l-lS FCC fractionator 650-665 bottoms reflux lines 16-17 FCC fractionator 600 side cut lines 18-20 530 21 FCC fractionator 390 bottoms reflux line We have found it most satisfactory to use the polyester couplants at the lower temperatures (i.e., 300-700 F.) and the polysulfone couplants at the higher temperatures (i.e., 600-1 F.)

The particular preference for the polysulfone resins as couplants is indicated by tests conducted at temperatures of 600l000 F. where the heated surface used was a laboratory hot plate. Excellent ultrasonic signals were received by the transducer thru the polysulfone resin couplant whereas poor results were obtained for polyester and other couplant materials at these high temperatures. Hence, the polysulfone resin is preferred for the higher temperatures and the polyester is satisfactory at temperatures in the 400600 F. range.

We claim:

1. Apparatus for the ultrasonic inspection of a body with a high surface temperature T, above 300 F. comprising, in combination, a transducer containing an ultrasonic wave generating source, a receiving device to receive the ultrasonic wave reflected from said body being inspected, measurement indicating means operatively connected to said receiving device, and, disposed between said transducer and said body being inspected, a polymeric couplant material having a glass transition temperature less than T, and a distortion temperature greater than both 300 F. and T, and which is deformable at T, to substantially fit the contours between said transducer and said body, said polymeric couplant material being selected from the group consisting of polysulfone resins and polyester resins.

2. In an apparatus for the ultrasonic inspection of a body with a high surface temperature T, above 300 F. which comprises a transducer containing an ultrasonic wave generating source, receiving means to receive the ultrasonic wave reflected from said body being inspected, measurement indicating means operatively connected to said receiving means, and a delay line for spacing said transducer from said high surface temperature body being inspected, the improvement which comprises the combination of said transducer, receiving means, measurement indicating means and delay line with a polymeric couplant material disposed between said delay line and said body being inspected which couplant material has a glass transition temperature less than T,, a distortion temperature greater than both 300 F. and T, is deformable at T, to substantially fit the contours between the end of said delay line and said body, said polymeric couplant material being selected from the group consisting of polysulfone resins and polyester resins.

3. The apparatus of claim 2 wherein said polymeric couplant material has a thickness of 0.001 to 0.2 inch.

4. The apparatus of claim 2 wherein said polymeric couplant material consists of a polysulfone resin having a thickness of 0.004 to 0.1 inch.

"H059 UNITED STATES PATENT OFFICE CERTIFECATE OF CORRECTION Patent No. 3, 7147, 398 Dated July 973 Richard P. Rathburn and Garth M. Stanton Inventor(s) n It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

F Preamble page, Column 1, line 13, delete "abandoned".

Column 1, lines 7 and 8, delete "now abandoned".

Signed and sealed this 18th day of December 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. RENE D. TEGTMEYER Attesting Officer I Acting Commissioner of Patents 

2. In an apparatus for the ultrasonic inspection of a body with a high surface temperature Ts above 300* F. which comprises a transducer containing an ultrasonic wave generating source, receiving means to receive the ultrasonic waVe reflected from said body being inspected, measurement indicating means operatively connected to said receiving means, and a delay line for spacing said transducer from said high surface temperature body being inspected, the improvement which comprises the combination of said transducer, receiving means, measurement indicating means and delay line with a polymeric couplant material disposed between said delay line and said body being inspected which couplant material has a glass transition temperature less than Ts, a distortion temperature greater than both 300* F. and Ts is deformable at Ts to substantially fit the contours between the end of said delay line and said body, said polymeric couplant material being selected from the group consisting of polysulfone resins and polyester resins.
 3. The apparatus of claim 2 wherein said polymeric couplant material has a thickness of 0.001 to 0.2 inch.
 4. The apparatus of claim 2 wherein said polymeric couplant material consists of a polysulfone resin having a thickness of 0.004 to 0.1 inch. 